1. Field of the Invention
The present invention generally relates to a data communication system and method, and more particularly to an on-vehicle type data communication system and method suitable for use in a network of components of an on-vehicle type AV (Audio Visual) system.
2. Description of the Related Art
Recently, on-vehicle audio systems have been changing from systems, which give the user only audio information such as music, to systems which allow the user to enjoy both audio and visual information. Systems with both audio and visual capabilities are known as AV (Audio Visual) systems.
The on-vehicle type AV system is provided with various kinds of audio and visual components. For example, the audio components may include a cassette tape deck, a radio tuner, a CD (Compact Disc) player, etc., while the visual components may include a TV (TeleVision) tuner, a navigation system, etc. Audio signals produced by some of these components are amplified by an amplifier and reproduced from loudspeakers in the automobile. Video signals produced by other components are displayed on a display unit mounted in the automobile. Today, the AV system components are controlled by means of digital technology, i.e., by respective controllers in the form of microcomputers.
The components of such an AV system are required to be controlled systematically in order to achieve a systematic operation of the components. To meet this requirement, the controllers of the respective components are connected to each other by a communication bus network, so that control data of the components will be transmitted to each other through the communication bus.
In the above mentioned network, the master unit transmits transmission data to a plurality slave units so as to allow all of the slave units to receive the transmission data substantially at the same time (which is referred to as "broadcast communication data", hereinafter), so that the slave units which have received the broadcast communication data perform operations corresponding to the broadcast communication data respectively. This type of communication in which the master unit simultaneously transmits the data to a plurality of slave units is called as the "broadcast communication".
In the above mentioned network system, the slave units are constructed such that, if the transmission data are transmitted to a specified one of the slave units from the master unit, the specified one of the slave units transmits an acknowledge signal back to the master unit so as to inform the master unit of the fact that the specified one of the slave units has actually received the transmission data. This type of communication in which the master unit transmits the data to a specific one of the slave units is called as the "ordinary communication".
However, in case of transmitting the above mentioned broadcast communication data, if the system is constructed such that each of the slave units transmits the acknowledge signal to inform the reception of the broadcast communication data respectively, the communication time will be necessary, which is substantially equal to the communication time required in the case of individually transmitting the transmission data to each of the slave units as in the ordinary communication, resulting in the loss of the benefit of the broadcast communication. Accordingly, the system is constructed such that the acknowledge signal are not transmitted with respect to the broadcast communication data.
This type of construction does not cause any problem as long as the reliability of the communication itself of the broadcast transmission data can be kept high.
However, in case of the on-vehicle type network, since it is necessary to place the system at the vicinity of the noise source such as an engine, it is difficult to keep the reliability of the communication itself high.
Therefore, even if there exists a slave unit to which the master unit transmits the broadcast communication data and which has failed to receive the broadcast communication data, the master unit cannot acknowledge this fact of failure in reception on the side of the slave unit, so that the master unit continues the normal processes. Thus, the reliability in the data communication is certainly degraded.